Conventionally, particulate streams that need to be heated or cooled are passed through rotating drums, over fluidised beds or along passageways defined between rows of parallel spaced plates. The plates, which are conveniently hung vertically, are hollow and coolant or heating fluid is passed through the interior of the plates. The particulate streams may pass vertically downwardly in their passageways and the coolant or heating fluid may pass upwardly through the plates. Heat exchange takes place through the surfaces of the plates which define the passageway walls. The distance between the plates, i.e. the passageways, may be varied depending on the type and size of the particulates to be processed.
The heat transfer efficiency in such plate-type bulk fluid heat exchangers, although improved over other known types, is limited if gases are used for heating or cooling. The plates are usually of stainless steel, dimpled and spot welded together via the dimples. The plates cannot withstand high internal pressures so that flow-through rates must be kept low, resulting in poor heat transfer depending on the characteristics of the heating or cooling medium. Thus these heat exchangers are essentially only suitable for heating or cooling by means of a liquid as a consequence of the high thermal capacity of liquids relative to gases. Liquids, however, have a limited operating temperature range in comparison with gases.